The variables whose names are specified shall be given the
readonly attribute. The values of variables with the readonly
attribute cannot be changed by subsequent assignment, nor can those variables
be unset by the unset utility. If the name of a variable is followed by
= word, then the value of that variable shall be set to word.
The readonly special built-in shall support the Base Definitions volume
of IEEE Std 1003.1-2001, Section 12.2, Utility Syntax
When -p is specified, readonly writes to the standard output the
names and values of all read-only variables, in the following format:
"readonly %s=%s\n", <name>, <value>
if name is set, and
"readonly %s\n", <name>
if name is unset.
The shell shall format the output, including the proper use of quoting, so that
it is suitable for reinput to the shell as commands that achieve the same
value and readonly attribute-setting results in a shell execution
environment in which:
Variables with values at the time they were output do not
have the readonly attribute set.
Variables that were unset at the time they were output do
not have a value at the time at which the saved output is reinput to the
When no arguments are given, the results are unspecified.
Some historical shells preserve the readonly attribute across separate
invocations. This volume of IEEE Std 1003.1-2001 allows this
behavior, but does not require it.
The -p option allows portable access to the values that can be saved and
then later restored using, for example, a dot script. Also see the
RATIONALE for export for a description of the no-argument and -p
output cases and a related example.
Read-only functions were considered, but they were omitted as not being
historical practice or particularly useful. Furthermore, functions must not be
read-only across invocations to preclude ``spoofing'' (spoofing is the term
for the practice of creating a program that acts like a well-known utility
with the intent of subverting the real intent of the user) of administrative
or security-relevant (or security-conscious) shell scripts.
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html